Alzheimer's Disease is a progressive, neurodegenerative disorder characterized by memory impairment and cognitive dysfunction. AD is characterized pathologically by the accumulation of senile (neuritic) plaques, neurofibrillary tangles, amyloid deposition in neural tissues and vessels, synaptic loss, and neuronal death. It is the most common form of dementia and it now represents the third leading cause of death after cardiovascular disorders and cancer. The cost of Alzheimer's Disease is enormous (in the U.S., greater than $100 billion annually) and includes the suffering of the patients, the suffering of families, and the lost productivity of patients and caregivers. As the longevity of society increases, the occurrence of AD will markedly increase. It is estimated that more than 10 million Americans will suffer from AD by the year 2020, if methods for prevention and treatment are not found. Currently, AD is estimated to afflict 10% of the population over age 65 and up to 50% of those over the age of 85. No treatment that effectively prevents AD or reverses the clinical symptoms and underlying pathophysiology is currently available (for review see Selkoe, D. J. Ann. Rev. Cell Biol., 1994, 10: 373-403).
There have been many theories relating to the etiology and pathogenesis of AD. These theories were either based on analogies with other diseases and conditions (e.g., slow virus and aluminum theories), or based on pathologic observations (e.g., cholinergic, amyloid, or tangle theories). Genetic analysis can potentially differentiate between competing theories. The identification of mutations in the β-amyloid precursor protein (β-APP) of individuals prone to early onset forms of AD and related disorders strongly supports the amyloidogenic theories.
Histopathological examination of brain tissue derived upon autopsy or from neurosurgical specimens in affected individuals reveals the occurrence of amyloid plaques and neurofibrillar tangles in the cerebral cortex of such patients. Similar alterations are observed in patients with Trisomy 21 (Down's syndrome). Biochemical and immunological studies reveal that the dominant proteinaceous component of the amyloid plaque is an approximately 4.2 kilodalton (kD) protein of about 39 to 43 amino acids. This protein is designated Aβ, β-amyloid peptide, and sometimes β/A4; referred to herein as Aβ. In addition to its deposition in amyloid plaques, Aβ is also found in the walls of meningeal and parenchymal arterioles, small arteries, capillaries, and sometimes, venules. Compelling evidence accumulated during the last decade reveals that Aβ is an internal polypeptide derived from a type 1 integral membrane protein, termed β-amyloid precursor protein (APP) (Selkoe, D. Physiol. Rev. 2001, 81, 741-766; Wolfe, M. J. Med. Chem. 2001, 44, 2039-2060). βAPP is normally produced by many cells both in vivo and in cultured cells, derived from various animals and humans. Several proteolytic fragments of APP are generated by proteinases referred to as secretases. A subset of these proteolytic fragments, designated β-amyloid peptide (Aβ), contains 39 to 43 amino acids and is generated by the combined action of β-secretase and γ-secretase. β-secretase is a membrane-bound, aspartyl protease that forms the N-terminus of the Aβ peptide. The C-terminus of the Aβ peptide is formed by γ-secretase, an apparently oligomeric complex that includes presenilin-1 and/or presenilin-2. Presenilin-1 and presenilin-2 are polytopic membrane-spanning proteins that may contain the catalytic components of γ-secretase (Seiffert, D.; Bradley, J. et al., J. Biol. Chem. 2000, 275, 34086-34091).
Multiple lines of evidence together strongly suggest that a reduction in brain Aβ levels will prevent the onset and progression of AD. First, Aβ is a major constituent of the parenchemyal plaques observed in all AD patients and the cerebral vasculature amyloid deposits observed in 90% AD patients (reviewed in Selkoe, D. Physiol. Rev. 2001, 81, 741-766; Wolfe, M. J. Med. Chem. 2001, 44, 2039-2060). These plaques are formed from the aggregation of soluble Aβ whose brain levels are highly correlated with the severity of AD neurodegeneration (McLean, C., Cherny, R. et al., Ann. Neurol. 1999, 46, 860-866). Second, mutations in three genes (APP, PS-1, or PS-2) that increase Aβ cause familial AD (FAD), where AD onset is accelerated by at least a decade. Included in the mutations that increase Aβ are chromosome 21 Trisomy that causes Down's syndrome. Third, transgenic mice that express one or more of the mutant FAD genes have increased Aβ levels, form parenchymal plaques and cerebral vascular deposits containing AD, exhibit memory deficits (Chapman, P.; White, G. et al., Nature Neurosci. 1999, 2, 271-276) and enhance neurofibrillary degeneration in mice that also overexpress mutant tau (Lewis, J.; Dickson, D. et al., Science 2001, 293, 1487-1491). Fourth, Aβ is toxic to cultured cells (Dahlgren, K.; Manelli, A. et al., J. Biol. Chem. 2002 277, 32046-32053), induces neurofibrillary tangles in mice with mutant tau (Gotz, J., Chen, F. et al., Science 2001, 293, 1491-1495) and interferes with long-term potentiation, a likely component of memory (Walsh, D., Klyubin, I. et al., Nature 2002, 416, 535-539 and references therein). Taken together, these data lead one skilled in the art to conclude that excess Aβ production and/or reduced Aβ clearance cause AD. From this it follows that reducing brain Aβ levels by inhibition of γ-secretase will prevent the onset and progression of AD.
In addition to AD, excess production and/or reduced clearance of Aβ causes cerebral amyloid angiopathy (CAA) (reviewed in Thal, D., Gherbremedhin, E. et al., J. Neuropath. Exp. Neuro. 2002, 61, 282-293). In these patients, vascular amyloid deposits cause degeneration of vessel walls and aneurysms that may be responsible for 10-15% hemorrhagic strokes in elderly patients. As in AD, mutations in the gene encoding Aβ lead to an early onset form of CAA, referred to as cerebral hemorrhage with amyloidosis of the Dutch type, and mice expressing this mutant protein develop CAA that is similar to patients.
A logical approach to reducing Aβ levels is to interfere with the action of the secretases that are directly involved in the cleavage of APP to Aβ. The β-secretase enzyme (BACE) is responsible for cleaving APP and forms the amino-terminus of Aβ, initiating the amyloidogenic pathway. The BACE enzyme is a transmembrane aspartyl protease and was described in the literature by several independent groups [see Hussain, I. et al., (1999) Mol. Cell. Neurosci., 14: 419-427; Lin, X. et al., (2000) Proceedings of the National Academy of Sciences of the United States of America, 97: 1456-1460; Sinha, S., et al., (1999) Nature (London), 402: 537-540; Vassar, R., et al., (1999) Science (Washington, D.C.), 286: 735-741; Walsh, D. M. et al., (2002); Wolfe, M. S. (2001); Yan, R. et al., (1999) Nature (London), 402: 533-537].
Removal of BACE activity in mice by gene targeting completely abolishes Aβ production [see Luo, Y., et al., (2001) Nature Neuroscience, 4: 231-232; Roberds, S. L. et al., (2001) Human Molecular Genetics, 10: 1317-1324].
BACE −/− mice also show no detectable negative phenotypes, suggesting that disruption of BACE-mediated cleavage of APP does not produce additional undesired effects. This demonstrates that a drug substance capable of inhibiting β-secretase activity should lower or halt the synthesis of Aβ and should provide a safe treatment for Alzheimer's disease.
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At present there remains an urgent need to develop pharmaceutical agents capable for effective treatment in halting, slowing, preventing, and/or reversing the progression of Alzheimer's disease. Compounds that are effective inhibitors of beta-secretase, that inhibit beta-secretase mediated cleavage of APP, that are effective inhibitors of Aβ protein production by beta-secretase, and/or are effective in reducing soluble Aβ protein, amyloid beta deposits or amyloid beta plaques, are needed for effective treatment in halting, slowing, preventing, and/or reversing neurological disorders related to Aβ protein production, such as Alzheimer's disease.